Method and system for supplying water to cooling towers

ABSTRACT

The present invention relates to a method and a system for controlling the circulation of water films on the exchange walls of an exchange surface of a cooling tower. It comprises combining a tank and a dispenser lip to ensure that the film has a specific thickness and that it adheres to the exchange wall right from the moment it starts to flow and repeatedly so, on each exchange wall. The exchange walls between air and water are horizontally sloping at a small angle, thereby ensuring that the water flows by force of gravity and, at the same time, limiting the speed increase on the plate to avoid the droplets from being pulled away by the airflow. The water films are recovered in recovery troughs perpendicular to the flow of the water films on the plates. These troughs are tilted horizontally, and enable the recovery of the water films without being crossed by the air flows. The air is blown in by nozzles which are interposed between the water flow plates in such a way that the airflow is in a counter-current flow or, if necessary, in a cross-current flow, relative to the water films thus enabling the evaporation of the water that cools the water flow on the exchange plates without forming liquid aerosols.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a §371 from PCT/FR2005/050398 filed May 31, 2005,which claims priority from FR 04/51128 filed Jun. 8, 2004, each of whichis herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to a method and system for supplying water tocooling towers.

Cooling towers are compact, energy-efficient devices for rejecting heatinto the ambient air. The essential physical process is that of waterevaporation in air that is distant from its moisture saturationconditions to an extent that varies according to local climaticconditions. Since the latent heat of water is very high, i.e. in therange of 2500 kJ/kg under atmospheric pressure, a low evaporation rateis needed to cool the flow of circulating liquid water. It has beenknown since 976 that cooling towers may be a source of vectorization ofpathogenic bacteria such as legionella. This vectorization takes placethrough liquid aerosols with a well-defined size ranging from 0.5 to 6micrometers, i.e. 0.5 to 6 10⁻⁶ m. Accordingly, an object of the presentinvention is to define a method and a system by which it is possible toeliminate these aerosols or micro-droplets.

PRIOR ART

Cooling towers are generally equipped with various devices to eliminatedroplet drift such as the droplet drift eliminators presented forexample in the U.S. Pat. No. 3,731,461 or the UK patent No. 2,206,683,each of which is incorporated herein by reference in its entirety.However, the measuring means used to measure the size of the aerosolsand their number are recent and not well known or, in the case of someof them, difficult to implement. Only recent devices using white lightdiffraction at 90 degrees can be used to count both the populations ofdroplets and their size, which is placed at the outlet of the coolingtowers. The manufacturers communicate the drift level in terms ofpercentage of the circulating water flow rate; typical values range from0.01% to 0.06%. This appears to be low but, when seen in relation to thecirculating flow rate, there are several tens of liters per hour thatare sent out in the form of aerosols having a size of some microns,representing values of more than several billions of micro-droplets perhour. Such numbers have been measured at the outlet of cooling towersprovided with droplet eliminator systems. The devices are not efficientin stopping micro-droplets sized between 0.5 and 6 micrometers. Thepresent invention therefore seeks to deal with the problem at source inpreventing the very formation of liquid aerosols. To this end, thepresent invention proceeds upon the desirability of developing a noveland comprehensive design for the flow of air and water in coolingtowers.

The cooling tower is formed chiefly by a water distribution system, apacking consisting of exchange surfaces for putting air and water intocontact, a ventilation system and a water recovery system.

The usual or improved devices, as presented in the U.S. Pat. No.4,579,692 or WO 99/30096 or WO 94/21366 (each of which is incorporatedherein by reference in its entirety) for the distribution of water onthe packing are spray devices, rotating booms or overflow systems thatshed water onto the packing. All these systems have the major defect ofgenerating aerosols even before the water flows onto the packing.Furthermore, in being concerned solely with increasing the air-watercontact surface, certain patents such as the U.S. Pat. No. 2,517,639 orNo. 3,652,066 (each of which is incorporated herein by reference in itsentirety) even claim an increase in the number of droplets formed byvarious devices to increase the air/water contact surface.

SUMMARY OF THE INVENTION

To prevent the formation of liquid aerosols or micro-droplets for eithercross-flow or counter-flow circulation of air and water in coolingtowers, this formation of liquid aerosols is prevented on threesuccessive portions of the flow of water in relation to the airflow:during the distribution of water, during the flow of water on theexchange surface and during the recovery of water at the end of theexchange surface or packing. The term packing is taken here in itsbroader sense of a solid surface providing for the efficient contact ofwater and air.

To this end, the present invention generates a film of water thatadheres to the wall of the packing, in checking the thickness and properdistribution on the surface. This is a first step in preventing theformation of micro-droplets. Then, it is necessary to check the waterflow regime on the exchange surface so that the height of the waveletsthat form on this free-boundary flow is low enough for the wavelets notto be clipped by the airflows. Finally, the water films should berecovered without being crossed by airflows.

The initial distribution of water on the surface is essential so as notto create aerosols of variable sizes during this distribution. Themethod devised as an embodiment of the invention is a method usingoverflow with controlled thickness and with film adhering to the wall.

Once the water film of homogeneous thickness has been distributedthroughout the width of the plate, the tilt of the plate, its surfacecondition, its hydrophilic properties or, on the contrary, itshydrophobic properties will determine the speed of water on the plate inconjunction with co-current or cross-current airflow circulation.Indeed, the water circulates by gravity and hence its motion isuniformly accelerated by gravity. This acceleration needs to becontrolled to limit the increase in the speed of water on the surfacewhich leads to the formation of wavelets. The Wallis criterion is usedto compute the relative speed thresholds of air and water leading to thepulling away of the droplets by the relationship U_(G)*+m√{square rootover (U_(L)*)}=C where U* is the non-dimensional speed, the indices Gand L respectively designate air and water and m is an empiricallydetermined parameter that depends on the surface condition of theexchange surfaces. The value of C makes it possible to know whether ornot the droplet pull-away conditions are fulfilled. Other moresophisticated computations taking account of the surface tension of thewater, the gravity, the wavelength of the wavelets, certainthermo-physical properties of air and water and of course their speedssimilarly lead to defining the droplet pull-away conditions. Thesecomputations and experimental devices have been used to verify the basisof the invention.

The present invention relates to a method for supplying water to acooling tower in which the supply is done by means of an anti-turbulencewater tank and a means such as a dispenser lip, wherein films of wateradhering to the exchange surfaces are generated in order to prevent theformation of liquid aerosols during the exchange between air and wateron these surfaces.

In accordance with an exemplary embodiment of the present invention, theexchange surfaces or plates are tilted by an angle, for example rangingfrom 2° to 10°, relative to horizontal, the value of this angle beingsuch that the acceleration of the water film on the exchange surfaces iscontrolled so that the speed of the film adhering to the surfacesprevents the clipping of the wavelets by the counter-current orcross-current air flows.

According an exemplary embodiment of the present invention, blower airnozzles are provided, comprising troughs inclined for example by anangle of 1° to 2° in a plane perpendicular to the flow of the water filmin order to collect this film without its being broken by the airflow,thus preventing the formation of droplets during the recovery of thewater films after they had been cooled by auto-evaporation in theairflows.

To ensure constant thickness of the water film on the plates or exchangesurfaces, in one embodiment the number of surfaces provided with waterdepends on the water flow rate. In this case, the supplied surfaces are,for example, each subjected to the same flow rate.

The invention also relates to a method for the production of watercooled by means of a cooling tower using a water distribution system andat least one exchange surface between a water flow and an airflow inwhich the water distribution system generates a flow in the form of awater film applied to the exchange surface, the values of thickness ofthe water film and of the relative speed of the water flow in relationto the airflow being chosen to prevent the formation of liquid aerosolsduring the exchange between the air and the water on these surfaces.

According to an exemplary embodiment of the present invention, thedistribution of water is obtained through overflow, by a distributionmeans providing for a homogeneous distribution of the water filmthroughout the width of the exchange surface.

In accordance with an exemplary embodiment of the present invention, thewater distribution system comprises an anti-turbulence water tank.

In accordance with an exemplary embodiment of the present invention, theexchange surface or plate is tilted by an angle, for example rangingfrom 2° to 10° relative to the horizontal, the value of this angle beingsuch that the relative speed of the flow of water in relation to theairflow remains below a threshold value starting from which aerosols getcreated.

According to an exemplary embodiment of the present invention, themaximum speed U_(L)* of the water film is determined by the formula:

U _(G) *+m√{square root over (U_(L)*)}=C,

where U_(G)* is the speed of the airflow, m is a parameter that is afunction of the exchange surface, and C is the value of the Wallacecriterion beyond which aerosols get created. The airflow is for examplegenerated by a distribution system situated at one of the ends of theexchange surface.

In accordance with an exemplary embodiment of the present invention,there are provided troughs inclined, for example, by an angle of 1° to2°, in a plane perpendicular to the flow of the water film in order tocollect this water film, without this film being in contact with theairflow, thus preventing the formation of droplets during the recoveryof the water films after they have been cooled by auto-evaporation inthe airflows.

In accordance with an exemplary embodiment of the present invention, toensure constant thickness of the water film on the plates or exchangesurfaces, the number of surfaces provided with water depends on thewater flow rate. In this case, the surfaces supplied are for exampleeach subjected to the same flow rate.

The flow of the water film is preferably laminar.

Other features and advantages of the invention should appear from thedescription of some of its embodiments, made with reference to theappended figures of which FIGS. 1 to 7 are diagrams illustratingembodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example, and notintended to limit the present invention solely thereto, will best beunderstood in conjunction with the accompanying drawings in which likecomponents or features in the various figures are represented by likereference numbers:

FIG. 1 depicts a schematic view of a cross-section of the waterdistribution system in accordance with an exemplary embodiment of thepresent inventions;

FIG. 2 is a comprehensive view of two superimposed exchange plates inaccordance with an exemplary embodiment of the present invention;

FIG. 3 is a view of the plate forming a first angle horizontally in thex, y plane in accordance with an exemplary embodiment of the presentinvention,

FIG. 4 is a partial and detailed view of the ends of the plates inaccordance with an exemplary embodiment of the present invention;

FIG. 5 is a view the plate forming an angle horizontally in the y, zplane in accordance with an exemplary embodiment of the presentinvention;

FIG. 6 illustrates the plates in accordance with an exemplary embodimentof the present invention; and

FIG. 7 illustrates a sectional view of the set of plates in accordancewith an exemplary embodiment of the present invention.

DESCRIPTION OF AN EMBODIMENT

This detailed description made with reference to the figures willprovide for a clear understanding of the invention. In accordance withan exemplary embodiment of the present invention, FIG. 1 gives aschematic view of a cross-section of the water distribution system 1which will be used for supplying each elementary exchange surface of thepacking such as the surface 4. The anti-movement water tank 2 receives afraction of the total water flow. Its dimensions and structure providefor an undisturbed flow that is properly distributed throughout thewidth of the elementary plate of the packing. The water comes out ofthis water tank to a dispenser lip 3. The aperture and the length ofthis dispenser lip 3 enable precise control of the thickness of thewater film 5 which is between some tenths of a millimeter and onemillimeter. The association of the water tank 2 and of the dispenser lip3 enables the distribution of the requisite fraction of the water flowover the exchange surface 4 throughout its width and with a definedthickness.

In accordance with an exemplary embodiment of the present invention,FIG. 2 provides a comprehensive view of two superimposed exchangeplates, 4 and 6, with the respective supplies from the water tanks 2 and9 which, in this implementation, are integrated into the thickness ofthe exchange plates 4 and 6. The exchange plates 4 and 6 typically havea thickness of the order of 5 mm. The water tanks 2 and 9 then have athickness of the order of 3 mm and the water film is poured with acontrolled thickness on the plate for by the dispenser lip 3. FIG. 2also shows one of the possible designs of the shape of the water tank 2having a section that is gradually reduced in the direction of the watersupply 7 to provide for an equally shared initial distribution of thewater flow throughout the width of the plate 4. The association of thewater tanks and of the dispenser lips on all the exchange plates of thepacking provide for the supply of the water by a film that adheres tothe exchange wall. This distribution system ensures the absence offormation of aerosols during the distribution of water.

FIG. 2 also shows a distribution element 10 formed by two thin plates 11and 12 that terminate in a conical shape to direct the airflow inparallel to the water flow and in a counter-current flow, in this caseon the plate 6. A cross-current supply of the air is also possible andwould have the same structure for water distribution using a water tankand dispenser lip and air distribution using interposed nozzles.However, as is well known, cross-current supply systems have lowerenergy efficiency. Advantageously, beehive structures, not shown, may beinserted into the mid-layer element of the distribution plates, formingthe air distribution system to obtain an essentially one-directional,eddy-free airflow.

In an orthonormal reference system x,y,z where x is the horizontal axisin the direction of flow of water on the plate, y is the vertical axisand z is the axis that forms a succession of horizontal planes with x,the plates form a first angle α between 2° and 10° and preferably around5° above the horizontal, in the plane x, y as shown in FIG. 3, in such away that the water supply system 1 formed by the water tanks and thedispenser lips is higher than the ends of the plates where the air isblown in by the air distribution structure. For a typical plate lengthof 1.7 m, the difference in level between the top and the bottom of theplate 4 is therefore about 15 cm enabling the speed of the water to beonly twice as high at the bottom of the plate as the initial speed atthe outlet from the distribution lip 3. This control of the effect ofacceleration of gravity on the water film is essential to maintain aslightly a rippled flow with a Reynold number below 1000 defining a flowregime in which the wavelets are low enough on the vertical so that theyare not clipped by the airflow, thereby preventing the formation ofdroplets and aerosols.

In accordance with an exemplary embodiment of the present invention,FIG. 4 is a partial and detailed view of the ends of the plates 4 and 6on which the water films 5 and 15 flow, and of the plates 11 and 12which are elements of the device 10 for the distribution of air in acounter-current flow with respect to the water film 5. It can be seenthat the end of the plates 4 and 6 is rounded to prevent turbulenceduring the change of direction of the water film. The plate 11 has atrough 13 which collects the water flow in a film 5 that has flowed onthe plate 4. This trough may advantageously have a section thatincreases in the direction of the slope y, z. Indeed, as shown in FIG.5, the plate also form an angle β of about 1° to 2° to the horizontal,this time in the plane y, z. This slope enables the recovery of eachelementary water flow flowing on each plate without crossing theairflow, thus preventing any formation of droplets by the blowing of airthrough the water flow. This principle of generalizing this waterrecovery system is shown in FIG. 4 where the plate 14 of the waterdistribution system itself also comprises a trough 16 used to recoverthe flow of water in a film 15.

Another exemplary embodiment of the present invention is shown in FIG.6: to avoid a case where the plates have two slopes, one in the plane x,y and the other in the plane y,z, only the slope in the plane x,y iskept and a part 19 is attached to the plate 11 and leans on the edge ofthe exchange surface 4. This attached part 19 forms a trough inclined inthe plane y,z. Furthermore, the section of the trough is gradually widerin the direction of the slope to take account of the increase in theflow associated with the gradual recovery of the water film 5. Forpractical reasons, this trough 19 may be integrated into the plate 11itself at the end of the course.

In accordance with an exemplary embodiment of the present invention,FIG. 7 shows a sectional view of the set of plates forming the packing17 with one of the water supply tubes 18 which supplies the inlets ofthe water tanks such as for example the inlets 7 and 8 shown in FIG. 2.Advantageously, several tubes, not shown, of the same type as the tubes18 may be positioned to alternately supply one in every two plates orone in every three plates or more if necessary. This provides thefollowing advantages: ease in the making of water inlet tap connectionson the supply tubes and above all the possibility of regulating thewater flow rate of the tower without modifying the thickness of thefilm. Indeed, for a nominal water flow rate representing 100% of theflow, all the exchange surfaces are supplied by all the supply tubes. Ifthe tower has three supply tubes and if the water flow rate has to bereduced by one-third, then one of the three supply tubes is closed by anad hoc valve and one-third of the plates are no longer supplied withwater. The other two-thirds are supplied with the same unit flow rate asearlier, thus making it possible to keep the same parameters of settingfor the dispenser lips and hence making it possible to have an even filmon each of the plates supplied.

In short, the invention relates to a method and system used to controlthe flow of water films on the exchange walls of an exchange surface ofa cooling tower by the association of a water tank and a dispenser lipensuring that the film or films have a defined thickness and adhere tothe exchange walls as soon as the flow of the film or films begins, thisbeing achieved repetitively on each exchange wall.

In the example, exchange walls between air and water are inclined to thehorizontal by a small angle, for example ranging from 2° to 10°, thusensuring the flow of water by gravity and at the same time limiting theincrease in speed on the plates so as to: (a) prevent the increase inthe speed on the plate to; and (b) prevent the droplets from beingpulled away by the airflow.

The recovery of the water films is done in recovery troughsperpendicular to the flow of the films of water on the plates. Thesetroughs are inclined to the horizontal by an angle equal, for example,to 1° to 2° and are used to recover the water films without beingcrossed by air flows.

Their air is blown in by nozzles interposed between the successivewater-flow plates in such a way that the air circulates in acounter-current or, if necessary, in a cross-current with respect to thewater films and thus enables the evaporation of the water which coolsthe water flow on the plates.

1. A method for supplying water to a cooling tower, comprising the stepsof supplying the water using an anti-turbulence water talk and adispenser lip; and generating films of water adhering to the exchangesurfaces to prevent the formation of liquid aerosols during the exchangebetween air and water on these surfaces.
 2. The method of claim 1,further comprising the step of tilting the exchange surfaces or plateshorizontally an angle, wherein the value of the angle is selected suchthat the acceleration of the water film on the exchange surfaces iscontrolled so that the speed of the film adhering to the surfacesprevents the clipping of the wavelets by the counter-current orcross-current airflows.
 3. The method of claim 1, further comprising thestep of tilting the exchange surfaces horizontally by the angle rangingfrom 2° to 10°.
 4. The method of claim 1, further comprising the step ofproviding blower air nozzles comprising troughs included by an angle ina plane perpendicular to the flow of the water film to collect the waterfilm without the water film being broken by the airflow, therebypreventing the formation of droplets during the recovery of the waterfilms after the water films have been cooled by auto-evaporation in theairflow.
 5. The method of claim 4, further comprising the step ofproviding troughs which are inclined by the angle of 1° to 2° in a planeperpendicular to the flow of the water film.
 6. The method of claim 1,further comprising the step of supplying the number of surfaces withwater based on the water flow rate to ensure constant thickness of thewater film on the plates or exchange surfaces.
 7. The method of claim 6,further comprising the step of subjecting each of the supplied surfacesto a same flow rate.
 8. A method for producing water cooled by a coolingtower using a water distribution system and at least one exchangesurface between a water flow and an airflow, comprising the steps ofgenerating a flow in the form of a water film applied to the exchangesurface by the water distribution system; and selecting thickness valuesof the water film and a relative speed of the water flow in relation tothe airflow to prevent the formation of liquid aerosols during theexchange between the air and the water on these surfaces.
 9. The methodof claim 8, further comprising the steps of providing a homogeneousdistribution of the water film throughout a width of the exchangesurface; and obtaining the distribution of water through an overflow ofsaid homogeneous distribution.
 10. The method of claim 8, furthercomprising the step of producing cooled water using the waterdistribution system comprising an anti-turbulence water tank.
 11. Themethod of claim 8, further comprising the step of tilting the exchangesurface or plate horizontally by an angle, wherein the value of theangle is selected such that the relative speed of the flow of water inrelation to the airflow remains below a threshold value starting from avalue at which aerosols are generated.
 12. The method of claim 11,further comprising the step of tilting the exchange surface or platehorizontally by the angle ranging from 2° to 10°.
 13. The method ofclaim 8, further comprising the step of determining a maximum speedU_(L)* of the water film by the formula:U _(G) *m√{square root over (U_(L)*)}=C, where U_(G)* is the speed ofthe airflow, m is a parameter that is a function of the exchangesurface, and C is the value of the Wallace criterion beyond whichaerosols get generated.
 14. The method of claim 8, further comprisingthe step of generating the airflow by a distribution system situated atone of the ends of the exchange surface.
 15. The method of claim 8,further comprising providing troughs inclined by an angle in a planeperpendicular to the flow of the water film to collect the water film,without the water film coming in contact with the airflow, therebypreventing the formation of droplets during the recovery of the waterfilms after the water films have been cooled by auto-evaporation in theairflow.
 16. The method of claim 15, further comprising providing thetroughs inclined by the angle of 1° to
 20. 17. The method of claim 8,further comprising the step of providing the number of surfaces withwater based on a water flow rate to ensure constant thickness of thewater film on the plates or exchange surfaces.
 18. The method of claim17, further comprising the step of subjecting each of the surfacessupplied to a same flow rate.
 19. The method of claim 8, furthercomprising the step of providing a laminar flow of the water film.